Direct Coupling with Zener Diodes

[kg]

Look at the graphs of the 12AX7, and as it swings up towards b+, it pretty much needs a positive grid if you want more than 2mA with a vp of 100volts.

No matter how you chose Rp1 and Rk2 values, the CF will start to reach a point where it's vp is too low for the amount of ik it is asked to pass to the load.

thankfully, at least someone gets it!

Besides, you've neglected the AC coupled load, which is significant b/c in use, there is signal, and the loading is quite substancial.

typically these cfs are driving something, a tone stack, an effects loop, etc, and that can be low impedance.

So in real circuits, there is no logic to match Rp1 and Rk2.

let's be frank here: there is no logic to doing that in virtual circuits, either. besides what i have read in this thread, i have never heard ANYONE claim any advantage to this sort of matching.

[jmaf]

Look at the graphs of the 12AX7, and as it swings up towards b+, it pretty much needs a positive grid if you want more than 2mA with a vp of 100volts.

thankfully, at least someone gets it!

Yeah and as that super magic grid flies towards +B, redelephant and kg decide to open a 12AX7 datasheet to check their amazing theories and they suddenly fall back, apalled to find out they had just defied the laws of physics!!!!



OMG, you guys are more than just super duper engineers, you are MAGICAL!

[redelephant]

Yeah and as that super magic grid flies towards +B, redelephant and kg decide to open a 12AX7 datasheet to check their amazing theories and they suddenly fall back, apalled to find out they had just defied the laws of physics!!!!

[ATTACH=CONFIG]13782[/ATTACH]

OMG, you guys are more than just super duper engineers, you are MAGICAL!

Dude. You don't get it! I'm actually trying to keep a sane discussion and correct your faulty ramblings in a polite and sympathetic way....

The fact that many tubes are not made to have their grids going positive is not b/c the grids won't go positive under the conditions shown in the graphs (as I tried to point out) but rather the grid wires are so fine they quickly get fried and the tube dies. So it's a tube manufacturer's recommendation, and that's all. Not a fact of the physics in a tube. As was pointed out earlier, the fine-gridded 12AX7 has been used in AB2 amps, just read here: EAR V-20 Amp Reviewed

Now why don't you study some tube datasheets and do some real engineering.

[Steve Conner]

Right, I'm going to build the DC coupled CF circuit in the lab on Friday afternoon, and measure grid current. We'll see who's breaking the laws of physics then.

[redelephant]

Yeah, just beware, with a 100kohm plate resistor for V1, the grid current will be in the microA range.

Also, the discussion regards the claim bout having to use matched Rp1 and Rk2...

This is getting to be a waste of time...like talking to a wall.

[kg]

Yeah and as that super magic grid flies towards +B, redelephant and kg decide to open a 12AX7 datasheet to check their amazing theories and they suddenly fall back, apalled to find out they had just defied the laws of physics!!!!

[ATTACH=CONFIG]13782[/ATTACH]

OMG, you guys are more than just super duper engineers, you are MAGICAL!

this is exactly why i resisted spending any time whatsoever in sharing my knowledge with you. obviously you have no interest in learning the truth, and instead are fixated on preserving your ego.

so why don't you keep on believing that tubes can't run with positive grid voltage and current, and i'll keep on dismissing everything you say as utterly unreliable and completely arbitrary, and we'll happily coexist.

[jmaf]

Right, I'm going to build the DC coupled CF circuit in the lab on Friday afternoon, and measure grid current. We'll see who's breaking the laws of physics then.

It is as simple as that - run the tests somewhere on planet Earth, not on a extraterrestrial lab where preamps have positive grids. I've ran all these tests...

1) If you get grid current, no signal is amplified on a 12ax7, it is positive cutoff at zero volts or by tube variation and error, a few milivolts above zero. Marshalls and Fenders surely don't do that.
2) Grid current you should get is around a few tens uA. Of course the tube is a diode then, it's unusable for amplification. Like I said all along, I must add, fearing to sound pedantic with these forum bullies trying to force feed their imaginary tubes.

If the tube is to be used as a diode, in fact, you can tie the grid to the plate. I've run all these tests, let's see how different your results are.

[jmaf]

this is exactly why i resisted spending any time whatsoever in sharing my knowledge with you. obviously you have no interest in learning the truth, and instead are fixated on preserving your ego.

so why don't you keep on believing that tubes can't run with positive grid voltage and current, and i'll keep on dismissing everything you say as utterly unreliable and completely arbitrary, and we'll happily coexist.

You just get personal whenever it suits you, I took lots of time to make sure I wasn't talking shit on this discussion. You can't amplify anything with a postive grid on a 12ax7, I hope you've learned that from this humble tech across the world. You're the one with the big ego, by the way, anyone can read this discussion and see how arrogant you were all along.

Edit: And from the moment you were arrogant and tried to ridicule my post in front of the forum, I stopped caring whether you ignore me...I wish it was otherwise and I tried to be sympathetic but you just kept insulting my intelligence. I have busy day ahead of me and I don't have time to mind this subject one minute more.

[redelephant]

It is as simple as that - run the tests somewhere on planet Earth, not on a extraterrestrial lab where preamps have positive grids. I've ran all these tests...

Whatever.

1) If you get grid current, no signal is amplified on a 12ax7, it is positive cutoff at zero volts or by tube variation and error, a few milivolts above zero.

Wrong. Besides are you talking about the CF or the common cathode stage?

Marshalls and Fenders surely don't do that.

Damn right. The values used simply don't allow for much grid current, a few microA perhaps, as stated before.

2) Grid current you should get is around a few tens uA.

Correct. But you don't seem to understand why.

Of course the tube is a diode then, it's unusable for amplification.

Wrong, but here again are you refering to the CF or gain stage?

Like I said all along, I must add, fearing to sound pedantic with these forum bullies trying to force feed their imaginary tubes.

...oh man!

If the tube is to be used as a diode, in fact, you can tie the grid to the plate. I've run all these tests, let's see how different your results are.

No shit, tube basics 101. I'm surprised you aknowledge that fact.

I've pretty much said what I can here. I'm done talking to a wall.

[imaradiostar]

There is an illustration in Merlin's first book showing a 12ax7 triode with the grid driven positive. He points out that the high driving impedance causes the signal to slightly clip but the signal is amplified perfectly. It's a physics thing. A 12ax7 driven from a very high impedance will clip like a diode. A 12ax7 driven from a much lower impedance will continue to amplify whenever is on its grid, even when the grid is positive.

JT

[Palladin]

RDH calls it damping - if you compare the rise over run for the data sheets (posted earlier) in both positive & negative grid situations, it should be clear how "slightly clipped" and how "perfectly amplified" the signal will be.
I'm thinking ... not so much; but we can all check that for grins.
Merlin offers a nice overview:
The Valve Wizard

[kg]

You can't amplify anything with a postive grid on a 12ax7,

quoted for posterity.

[jmaf]

quoted for posterity.

What posterity? You waitin' for someone to cover your back? I'm in this alone you know... you already got the cronnies up there helping you out....

[Steve Conner]

Oh no! It's jmaf against a mysterious cabal of alien magical engineers.

To tell you the truth, I've never worked with the DC coupled CF and am not sure of the result myself. I think that with the gain stage/CF unit as seen in the 5F6A and a million Marshalls, the top of the clipped wave will be rounded off, or at least dragged down, compared to a triode stage overdriven by itself. This will be caused by grid current in the CF section.

Changing the plate and cathode resistors will alter it somewhat, but matching them won't do anything magical. I think one of the resistors in jmaf's circuit must have been badly out of tolerance.

When you overdrive the 5F6A circuit connected to a tone stack, things get really weird. The CF cuts off on negative half cycles, leaving the bass capacitor to discharge through the cathode resistor. I will try this too time permitting.

[mooreamps]

Why would you bother to write this? This isn't going to make the thread any better and your just isolating yourself by insulting other long time members. How is it you thought this would make anything better, help anyone, further your own goals, serve your god or do anything useful at all?

Not a rhetorical question.

Because "they" isolated me when I first started writing about using fixed bias in preamps going back about 6 years ago. This is the same kind of ground I've already covered... Why should I help them ?

-g